Knock, knock. / Who's there? / Pizza.

Knock, knock.
Who's there?
Pizza.
Pizza who?
Pizza really great guy!

This is a knock-knock joke. The critical element of the joke that makes it funny is the pun centered around the word "Pizza". "Pizza" sounds like "Pete's a" as in "Pete is a". So, the punchline really is "Pete is a really great guy".

I give this joke 10 chances in 9 of having been created by a guy named Pete.

That would be so like him.